Long-distance telecommunication is really important, and currently
crucially dependent not only on the state of the global economy, but
also on operationally centralized telecommunications companies, large
telecommunications structures that are easy to bomb, and undersea
cables. What's the minimal digital telecommunications infrastructure
that would usefully enable intercontinental communication?

Possible communications modes
-----------------------------
### Shortwave radio ###
Shortwave radio (1.5MHz-30MHz) reflects off the ionosphere and can be
received intercontinentally. This includes a couple of ISM bands in
which unlicensed operation is permitted internationally:
13553–13567 kHz (22 meters) and 26957–27283 kHz (11 meters).
Wikipedia's "High frequency" article says, "The maximum usable
frequency regularly drops below 10MHz in darkness during the winter
months, while in summer during daylight it can easily surpass 30MHz."
Wikipedia's "Skywave" article says, "Signals of only a few watts can
sometimes be received many thousands of miles away as a result." One
hop off the ionosphere gets your signal up to 3500km.
This means that, in theory, you could set up a single unlicensed radio
station providing communications services to you in most of a
7000-kilometer-wide circle. Something over a dozen such receivers
could provide you with global coverage. Of course, you'd have to deal
with interference: both decoding the received signals in the face of
interference, and not generating so much interference to other users
of the spectrum that they start to interfere with you in ways other
than radio communication.
(Transmitting in the 22-meter band, especially using time-domain radio
with low-power code division multiplexing and a low duty cycle, would
also make it practically difficult to locate the radio transmitter;
and of course the receiver would produce even less evidence of its
presence.)
#### Possible bit rates for shortwave ####
Shannon's Theorem, aka the noisy-channel coding theorem, has no
minimum signal strength below which communication bandwidth is zero;
the Shannon-Hartley limit is B log (1 + S/N), where B is bandwidth in
hertz, and S and N are the power of the signal and noise. In the
"power-limited regime", when S/N is very small, this is linear in S/N.
A somewhat counterintuitive result here is that, practically speaking,
your bit rate doesn't depend on frequency selectivity much at all ---
if you use twice as much of the spectrum, you double B, but you also
probably double N. This means you can choose your bandwidth to
satisfy other considerations, such as minimizing interference with
other users of the band, who are mostly still using frequency-division
multiplexing, or minimizing device cost.
So, supposing you can send one bit per second in the HF band using one
watt over a few thousand kilometers, you can probably send about one
bit per 1000 seconds using one milliwatt, or one bit per day using 12
microwatts, and this regardless of whether your signal is 0.1kHz wide,
14 kHz wide, or 28.5 MHz wide.
Is one bit per second about right? Digital Radio Mondiale in the
shortwave bands uses at least 6 kilobits per second, but that doesn't
tell us much, since this could be slammed through by using hundreds of
kilowatts. What we really need to know is how much background noise
there is in these frequency bands, in an absolute sense, or how many
words per minute people can send and receive with CW Morse code over
what distance and what power. One bit per second would be about 1.2
"words per minute".
It seems like the keyword for this kind of thing in current amateur
radio operation is "QRP operation", which means around 5 or 10 watts
or less. QRPers consider anything below 1 watt as "extremely low
power", or QRPp, and they consider a thousand miles per watt as a
difficult benchmark to meet. PSK31 is the digital code most commonly
used at present for this kind of thing, and it operates at 31 baud and
a bandwidth of 31.25Hz; it does a 180-degree phase shift during an
amplitude null to represent a zero bit after encoding the digital
signal into Varicode, a huffman code for ASCII. There are 10-baud and
5-baud variants in use. PSK31 is commonly used down to the 160-meter
band.
If we figure that talking a thousand miles (1609 km) on a watt already
puts you into the power-limited regime, and guess that you can get
some 10 bits per second out of it, then using skywave bounce, you
should be able to talk N thousand miles at 10/N bits per second. For
example, you should be able to talk 16000 km, about a third of the way
around the world at one bit per second.
### Lasermoon moonbounce communication ###
You could illuminate the moon with a rapidly modulated laser at night.
The moon's albedo is about 12%, it covers about half a degree of arc,
and you can get cheap 5mW semiconductor laser pointers with divergence
of less than that, indeed closer to a tenth of a degree, so that all
of their power falls on the moon. Similarly, an inexpensive lens
focusing the moon's image onto a photodiode (which is faster than a
phototransistor) can easily give you better than an arcminute of
selectivity. A square arcminute is about 85 nanosteradians, so this
corresponds to about 80 dBi of "antenna gain" on the receiving end,
but focusing more tightly than the original laser pointer spot isn't
useful, so you only get about 60 dBi in practice.
Against this you must weigh the fact that most of the light falling on
the moon is reflected off into space. You can probably reasonably use
a one-meter-diameter plastic Fresnel lens, but you're about 380
megameters from the moon. This means that the light signal you get is
about 176 dB weaker than the light signal sent from the moon, which is
presumably most of the 500μW or so of light emitted from a 5mW laser
pointer, minus about 9dB for the low albedo. This means you're trying
to detect a signal of some -188dBm, which is difficult but feasible.
It's actually considerably more feasible than doing the same trick
with radio waves, due to the higher "antenna gain".
I think common laser diodes have a Q factor around 1000 --- their
light is mostly contained in a range of wavelengths of less than a
nanometer --- which means you could improve your SNR by up to about
30dB by filtering the moon's image through a dichroic filter.
This leaves the question of how much noise you're dealing with in the
first place, particularly important if you might have significant
"crosstalk" from brightly-illuminated parts of the moon, say because
your camera is imperfect. Sadly, I have very little idea how bright
earthshine is, which is kind of the question.
### Earth-mode communications ###
Ultra-low frequency radio --- 300 Hz to 3000 Hz --- can propagate
through the earth itself; there are apparently lots of NATO papers
from the 1960s about using this for military purposes. As far as I
can tell, though, this involves running antenna cables a distance
comparable to half the wavelength (100 km or more) or to the distance
you want to communicate. So I think this may be impractical.
### Balloon radio ###
Getting a balloon to 90km altitude is straightforward, at which point
it has a line-of-sight radio path to anything within 9.57 degrees of
arc (acos(r/(r + 90 km)), where r is 6371 km, the radius of the Earth)
on the Earth's surface, about 1000 km; at this point it could quite
reasonably communicate at gigabits per second using laser or
highly-directional ultrawideband microwave to any of those points, if
it can figure out how to align its transmitter and receiver.
Furthermore, two such balloons will have a line-of-sight if they're
less than twice the angle apart, or 2000 km. Twenty such balloons
could ring the earth along a great circle, so a frequency-four
geodesic sphere would suffice to provide global coverage with a bit
under 200 balloons.
On the other hand, getting balloons to stay up there would be quite a
challenging project. In fact, just getting them to work at all while
they're up there is no walk in the park.
### Air mail ###
In 1998, Laima, an Aerosonde, flew successfully across the Atlantic on
5.7 liters of fuel in 27 hours. Aerosondes weigh about 13 kg, about
the same as a large swan. One micro-SD card weighs about 500mg and
can currently hold 64 GB of data, so a 2-kg payload would hold 256 TB.
This would amount to a 21 Gbps transatlantic data connection.
In 2003, the 5kg Spirit of Butts Farm made more or less the same
3000km flight on under a liter of fuel, in just under 39 hours.
On one hand, these bandwidths are orders of magnitude higher than the
bandwidths reasonably achievable by the other technologies mentioned
above. On the other hand, the latency is also some five orders of
magnitude higher, and also, this is a very unreliable way of
transmitting data --- it is likely to result in the loss of the
aircraft and its payload on a regular basis.
A lower-tech approach for north-south communication would be to attach
high-capacity store-and-forward radio-communication nodes to
long-lived migratory birds. When they came into communication range
of other nodes, they would exchange messages via short-range,
high-bandwidth radio.
### Neutrino beams ###
Particle accelerators can generate large quantities of neutrinos in a
beam, by accelerating large quantities of pions or kaons to
relativistic speeds and letting them decay; if the neutrino beam is
directed at the earth, most of them will penetrate to the other side.
Unfortunately, detecting a change in neutrino flux is very difficult,
because they can easily penetrate things like the earth. It's
feasible using thousands of photomultiplier tubes and tens of
thousands of tonnes of water or other transparent liquid deep
underground, like the Super-KamiokaNDE and the Sudbury Neutron
Observatory; such observatories see on the order of one natural
neutrino decay per few days per thousand tons of matter included.
This is clearly a feasible technique for communication, but it costs
tens to hundreds of millions of dollars (which is to say, hundreds to
thousands of person-years of work) and the result should be
communication with bandwidth measured in fractional bits per day. If
it were possible to increase the number of neutrinos by several orders
of magnitude, you could increase the bit rate substantially, and in
the limit, you could have lower latencies for long-distance
communication than communication methods for which the earth is an
obstacle.
### Sound in air or water ###
Low-frequency sound can propagate for hundreds of kilometers in air,
which is the principle behind the West African talking drums, which
were used for long-distance digital communication for several
centuries, up to the mid-20th century. I think you can get up to tens
of bits per second this way. The talking drums could transmit several
kilometers (8 km, says Wikipedia), but not tens or hundreds of
kilometers. Whistled speech can cover one to two km, and has in
theory a higher bit rate.
High-frequency sound is unusably attenuated by air in a few
kilometers, but can cover thousands of kilometers in water, where
whales use it; it also travels above 1km/sec. There's the additional
advantage that sound in the ocean tends to remain a few hundred meters
from the surface, in a zone called the SOFAR zone or Deep Sound
Channel, so its intensity falls off as 1/d instead of 1/d² over long
distances.
### Telegraph towers and cloud-bounced signal lamps ###
A 1.6-km-tall mountain, such as Sandia Peak in Albuquerque, has a line
of sight to some 140 km distance on a smooth-spheroid earth, and
perhaps 100 km in real life. Even a 100-meter-tall tower can see for
36 km. This principle was used in semaphore telegraphs throughout
Europe in the decades before the development of the electric
telegraph: a line of fortified stone towers at 30-km intervals
provided about one bit per second. If you put a cheap laser pointer
on the tower, you can transmit over this line of sight at megabit
speeds or better.
Of course, you can also use a reflecting mirror to reflect sunlight
instead of a laser; such a "heliograph" can transmit up to 300km under
ideal conditions or 50km under normal conditions, using small,
hand-sized mirrors. Bit rates depend on how fast you can open and
close some kind of "shutter", but at least hundreds if not thousands
of bits per second should be attainable with inexpensive LCDs.
As suggested in the Lasermoon section, you can do this same kind of
long-distance communication even without going up on the mountain
yourself; it's sufficient to illuminate it from afar. Naval
communication with Aldis lamps uses this trick to communicate by
illuminating the bases of cloud formations, which enables
communication at much greater distances; in theory, you could use
cirrocumulus or cirrostratus clouds at their altitude of some 5–12 km
to communicate to a station 500–800 km away, or perhaps even the
23-km-high tip of a cumulonimbus cloud to communicate with a station
1000 km away. This is more practical than using the moon, since you
only have to detect light diffusing from the cloud 500 km from you
instead of 380 000 km from you, giving you 60 dB less path loss in the
cloud-to-you path than in the moon-to-you path. (As before, I think
you can focus light onto the cloud well enough that effectively all
the light you transmit reaches the cloud, so you effectively don't
have any path loss.)
Useful bit rates
----------------
Why would it matter if you could transmit a millibit per second?
Well, one bit can still be a crucially useful thing to be able to
transmit. "I'm alive," for example, or "The killers weren't al
Qaeda." But even a Tweet is only 1120 bits, so a bit per 1000 seconds
would let you you transmit a Tweet every two weeks. The Annals of
Spring and Autumn, 春秋, covers 541 years in 16000 characters; if each
were 16 bits, that would be just over one bit per day on average. The
Chappe semaphore line system transmitted about one "symbol" per minute
out of 92 possible symbols, giving a bandwidth of about 0.11 bits per
second; it was credited with giving Napoleon a decisive military
advantage.
### One bit per second ###
Consider the higher bandwidth of one bit per second, which I've
hypothesized above requires about a watt of power over
transcontinental shortwave. Two bits is roughly one letter of
compressed ASCII text [0], so this is about 7000 words of text per
day; one to ten Wikipedia articles per day, for example, or a fairly
in-depth summary of daily international news, or a couple of hours of
writing email per day, or a book per month. Furthermore, it's fast
enough to permit someone to write a line of text in a chat system,
transmit it, and get back a response from someone else within a couple
of minutes.
[0] bible-pg10.txt is 824 146 words and 4 452 069 bytes; its .bz2 is
999 906 bytes, 1.80 bits per uncompressed byte; compressors other than
bzip2 may be more efficient; but extremely high compression ratios may
only be achievable with the transmission of large amounts of data.
For example, if I split the same document into 64KiB chunks and bzip2
each one independently, the result is 1 214 055 bytes, 2.18 bits per
uncompressed byte.
One bit per second, then, is a sufficiently rapid connection to permit
real participation in the intellectual life of an invisible college;
but it's less information than a person can read. Furthermore, with a
broadcast medium such as radio, it's reasonable to transmit the same
information to many recipients at a time, Usenet-style.
Commercially speaking, even one bit per second, or for that matter a
thousandth of a bit per second, at low latency is enough to make you a
hell of a lot of money in commodity arbitrage, if your competitors
don't have access to equally fast communication. A price to three
significant figures is only ten bits, and it's straightforward to
encode names for widely-traded commodities in 30 bits or less. (This
is why the stock ticker was such big business in the late 19th and
early 20th century.)
### Ten bits per second ###
Ten bits per second would give you some 70 000 words of text per day,
which is almost as much as you could read in real time; and it would
be fast enough to permit real-time hypertext navigation with a
stateful two-way protocol. (If you're transmitting document contents
containing the IDs of other documents in one direction, while
transmitting the IDs of requested documents or document parts in the
other direction, each ID will typically be within the last thousand
IDs transmitted, so it can be encoded in ten or twenty bits.) Ten
bits per second at 2.5 bits per character works out to 240 characters
per minute, or 48 words per minute, so it's at least theoretically
possible for one person to type faster than ten bits per second on a
sustained basis; but it's fast enough that one-to-one text chat would
be more or less real time.
### A hundred bits per second ###
A hundred bits per second would give you some 700 000 words per day,
an entire book. This is enough that one person can no longer read all
of it, if it's text, and eliminates the need for a stateful protocol
for real-time hypertext --- although full HTTP might still be
excessive. It's about 480 words per minute.
Acquiring one new book per day was an unachievable dream for nearly
all individual scholars and the majority of libraries in most of the
age of print. If the books were new cheap paperbacks, it cost about
US$1800 per year toward the end of the 20th century, within the means
of most of the population of the developed world, but new hardbacks
pushed the cost up by an order of magnitude, to US$18000 per year. So
a hundred-bit-per-second connection to a large data store could beat
access to printing presses for all but the wealthiest.
If one person were trying to consume a hundred bits per second, they
would probably want to do much of it in the form of still images. A
high-quality color JPEG file typically uses about four bits per pixel,
and black-and-white needs about half that. So a hundred bits per
second is on the order of 100 low-quality pixels or 25 high-quality
pixels per second. A low-quality 320×200 color image might download
in 21 minutes.
As of early 2012, the English Wikipedia articles without pictures
total 10 399 030 471 bytes in a compressed Zim file. At a hundred
bits per second, this file would take 26 years to transmit.
For much of the twentieth century, most Telex systems ran at 45.45
bits per second.
### A thousand bits per second ###
A thousand bits per second is 7 000 000 words per day, some ten books.
At this speed it's possible to receive text you don't intend to read
yourself, just in case you want to read it later; but it's also fast
enough that you don't need to if the connection is reliable. You
could download the Wikipedia archive I mentioned above in 2.6 years.
It's also at the minimal lower limit where you can do real-time voice
transmission. Many voice codecs (from early LPC work in the 1970s to
HVXC and Speex today) have modes between 2 and 3 kilobits per second.
I think transmitting intelligible speech at 1kbps is feasible, but
only barely, and I don't know if it's been done.
At a thousand bits per second, you can completely kick the ass of
printing-press-based communication systems, both providing instant
hypertext access to faraway data stores and accumulating the
equivalent of thousands of locally-stored books per year, plus, of
course, email.
### Ten thousand bits per second ###
This is the speed at which much of Fidonet operated.
Possible communications hardware and algorithms
-----------------------------------------------
The very low power levels involved suggest that it might be possible
to do these ultraslow transmissions with extremely steampunk
equipment, or not even that. For example,
for radio communication, perhaps you could trigger a
static-electrical spark from a Wimshurst Influence Machine at
intervals driven by holes in a rotating glass sphere, firing an
electrical impulse into a dipole antenna. But a Wimshurst machine can
produce more than a watt of power; a simpler generator, such as your
hair and a balloon, might be sufficient. You just have to time your
sparks correctly to make them interpretable by your (presumably much
more capable) receiver, and successfully couple them into your
antenna, maybe with a low-Q resonator containing a coil and a Leyden
jar.
(In theory, maybe you could decode such a low-bandwidth signal by
hand, too, but it's not obvious to me how you'd do it.)
(The earliest non-spark-gap radio, the Poulsen arc, used a carbon arc
as a negative-resistance device to produce a dynatron oscillator; I
think you could also use just about any gas discharge tube, such as a
fluorescent lamp tube or even a fluorescent lamp starter.)
Of course, multiple ultraslow radios could form a very slow
packet-switching network.
Moving further into the steampunk and cargo-cult direction, you could
imagine a shorter-range purely passive radio transmitter, like a sort
of switchable analog RFID tag. The "receiver" would wait for the
transmitter to be energized by electromagnetic pulses produced by
nearby lightning strikes; then it would measure a variable set of RF
resonances in the device. A Leyden jar might have a nanofarad of
capacitance, and a variable inductor with up to about a henry of
inductance can be fairly easily constructed by hand, the two forming a
tank circuit with a resonant frequency of as little as 5kHz and a Q of
perhaps 100; a large number of such resonant circuits connected to an
antenna will "ring" for tens or hundreds of oscillations when
subjected to a radio impulse. If their frequencies are chosen in
accordance with some kind of error-correcting code, a distant detector
will eventually be able to determine what those frequencies are, after
detecting enough impulse responses. Multiple receiving antennas could
provide enough phase diversity to do beamforming.
Such a device is fairly inefficient, since most of the energizing
pulse energy will not coincide with a resonant frequency of the
transmitter and will thus simply reflect from the antenna. If you
could store the pulse in some kind of device that allowed you to use
it to continuously energize the resonators until it dissipated, you
could use more of it. For example, with a semiconductor diode, you
could store much of the pulse in a capacitor, and use that to power
active oscillator circuits; or if you could circulate it through some
kind of delay line, such as a piezoelectric acoustic delay line, you
could use any nonlinear but nondissipative circuit element to shift
some of the remaining pulse energy into resonable harmonics or
subharmonics on each circulation through the system.
Pulse radio would allow for relatively reasonable decoding electronics
and algorithms: while your average power might be, say, 100
milliwatts, if you're only transmitting for 100 microseconds out of
every minute, that transmission would be at 60 kilowatts. (If it's a
single 100-microsecond pulse, it would be a six-joule pulse; into a
63-ohm antenna impedance, it would be a momentary current of 31 amps,
driven by 2000 volts, which could be stored in 3 microfarads, so this
wouldn't require any particularly exotic circuit design.) Detecting
the pulse at even a very substantial distance should not require
particularly sensitive or highly linear analog electronics; the
remaining task, then, is to figure out which subset of the detected
pulses represent a decodable message.
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